Polyesters such as, for example, polyethylene terephthalate and polybutylene terephthalate, generally referred to as "polyalkylene terephthalates", are a class of important industrial polymers. They are widely used in thermoplastic fibers, films, and molding applications. They are generally produced by transesterification of a dialkyl terephthalate ester with a glycol or by direct esterification of terephthalic acid with the selected glycol followed by polycondensation. A catalyst is used to catalyze the esterification or polycondensation. Frequently an organic titanium compound is used as the catalyst of choice for the esterification and/or polycondensation. Certain phosphorus compounds may also be added during or after the esterification and polycondensation steps as catalyst modifiers.
An important group of polyesters are those polymers made with adipic acid and a glycol, or copolymers made with a combination of adipic acid and terephthalic acid, with a glycol such as 1,4-butanediol.
A titanium compound is often used as catalyst for the production of polyester. However, when an organic titanium compound is used as the polycondensation catalyst with adipic acid-containing monomers, the resulting polymer or copolymer is typically intensely colored, generally red. This is probably attributed to a complex formed between the titanium and the adipic acid or adipate ester.
For example, WO 99/28,033 discloses an organometallic compound suitable for use as catalyst for the preparation of an ester comprising the reaction product of an orthoester or condensed orthoester of titanium, zirconium, or aluminum, an alcohol containing at least two hydroxyl groups, an organophosphorous compound containing at least one P--OH group, and a base. It does not disclose or suggest a process useful for preventing formation of color during the polycondensation of adipic acid-based polymers or copolymers. JP 11236,442A discloses a method for producing a polyester consisting of adipic acid and 1,4-butanediol using a titanium compound as condensation catalyst to produce a molten polyester and, adding after the polycondensation step a pentavalent phosphorous compound containing at least one hydroxyl group. Since this method requires the titanium and adipate compounds to be in contact before addition of the phosphorus compound, it is only partially effective in preventing formation of a red color. JP 04080932B discloses preparation of adipic acid copolymers comprising esterification or transesterification of terephthalic acid or its derivative and butanediol in the presence of a titanium compound and, thereafter, polycondensing the resultant reaction product in the presence of a titanium compound and a pentavalent phosphorous compound. JP 60147430 discloses a method of producing a polyester by esterifying terephthalic acid, adipic acid and 1,4-butanediol in the presence of a titanium compound, and then polycondensing the resulting reaction product in the presence of a titanium compound and a pentavalent phosphorus compound. Adding the phosphorus compound as a separate compound before the polycondensation step tends to cause deactivation of the titanium catalyst.
Therefore, there is a need to develop a process for producing polyester, especially an adipic acid-base polyester or copolymer thereof, with little or no discoloration.